Abstract: Based on the theory of Buckley-Leverett of the oil being displaced by water in non-slug flooding and the laboratory test of an expert in oil field development named ЭФPCO of former USSR and the empirical formula of caculating reserves of Tong Xianzhang who was an academician of the Chinese-Academy of Engineering. Combining the production data with reservoir fluid data and its parameters, this paper deduced a new method to get the initial oil saturation and current oil saturation in a reservoir. Using the method of the paper's introduction, we get initial oil saturation and current oil saturation and compare initial oil saturation with experimental result, the error of calculation is less than 1.45%.